Containers have long been employed to store and transfer perishable food prior to presenting the food at a market where it will be purchased by the consumer. After perishable foods, such as meats, fruits, and vegetables, are harvested, they are placed into containers to preserve those foods for as long as possible. Maximizing the time in which the food remains preserved in the containers increases the profitability of all entities in the chain of distribution by minimizing the amount of spoilage.
The environment around which the food is preserved is a critical factor in the preservation process. Not only is maintaining an adequate temperature important, but the molecular and chemical content of the gases surrounding the food is significant as well. By providing an appropriate gas content to the environment surrounding the food, the food can be better preserved when maintained at the proper temperature or even when it is exposed to variations in temperature. This gives the food producer some assurance that after the food leaves his or her control, the food will be in an acceptable condition when it reaches the consumer.
Modified atmosphere packaging systems for one type of food, raw meats, exposes these raw meats to either extremely high levels or extremely low levels of oxygen (O.sub.2). Packaging systems which provide extremely low levels of oxygen are generally preferable because it is well known that the fresh quality of meat can be preserved longer under anaerobic conditions than under aerobic conditions. Maintaining low levels of oxygen minimizes the growth and multiplication of aerobic bacteria.
One example of a low-level oxygen system is disclosed in U.S. Pat. No. 5,698,250 to DelDuca et al. ("DelDuca"), which is incorporated herein by reference in its entirety. FIGS. 1 and 2 of DelDuca are reproduced herein as FIGS. 1 and 2. Referring to FIGS. 1 and 2, DelDuca discloses a modified atmosphere package 10 including an outer container 12 composed of a oxygen barrier material and an inner container 14 composed of a material substantially permeable to oxygen. The inner container 14 is preferably comprised of a polystyrene foam tray 16 and a stretch film wrapping 18. The tray 16 contains a retail cut of raw meat 26. An oxygen scavenger 28 is located between the inner container 14 and the outer container 12.
To create a modified atmosphere in the package 10, DelDuca employs the following method. First, the meat 26 is placed within the inner container 14, and the inner container 14 is then sealed. Second, the inner container 14 is inserted into the outer container 12. Third, without using any evacuation, the outer container 12 is flushed with an appropriate mixture of gases, such as 30 percent carbon dioxide and 70 percent nitrogen, to remove most of the oxygen from the outer container 12. Fourth, the outer container 12 is sealed. Fifth, the oxygen scavenger 28 is activated and used to absorb any residual oxygen within the package 10. The DelDuca method relies upon activation of the oxygen scavenger 28 to quickly absorb the residual oxygen.
FIG. 2 identifies four oxygen sources, or zones, that exist within the package 10. Zone I is the oxygen volume between the outer container 12 and the inner container 14; zone II is the oxygen volume within the inner container 14; zone III is the oxygen volume within the cells of the foam tray 16; and zone IV is the oxygen volume within the meat 26, which is believed to be minimal with the exception of ground meats. The oxygen scavenger 28 is located in zone I.
In the above-described DelDuca method, the step of flushing the outer container 14 lowers the level of oxygen within the package 10 to about 0.05 to 5 percent. At such oxygen levels, especially at the lower end of the above range (0.05 to 2 percent), metmyoglobin can form very quickly. Metmyoglobin is a substance that causes meat to change to an undesirable brown color. Metmyoglobin forms very slowly at oxygen levels above 2 percent and below 0.05 percent but very quickly between these oxygen levels. Accordingly, it is important to pass the meat located in zone II through the pigment sensitive oxygen range (0.05 to 2 percent) very quickly, e.g., less than about two hours. Although DelDuca contemplates flushing the inner container 14, existing technology generally will not flush zone II down below the pigment sensitive oxygen range. Therefore, even if the inner container 14 is flushed, the oxygen level in zone II must still be passed quickly through the pigment sensitive oxygen range.
In DelDuca, after the outer container 12 is sealed, oxygen remaining in zone II (within the inner container 14) passes through the substantially, but not 100 percent, permeable material of the inner container 14 and is rapidly absorbed by the activated oxygen scavenger 28 in zone I. The faster the rate of oxygen egress from zone II into zone I, the faster the oxygen level in zone II can be passed quickly through the pigment sensitive oxygen range. The present invention is directed to techniques for improving the rate of oxygen egress from zone II into zone I. In addition, the present invention is directed to techniques for directly absorbing oxygen in zone II before the oxygen passes into zone I.